I am reading the instructions on conkeror site (and elsewhere) on how to install conkeror on Ubuntu (I am using Ubuntu 12_04 LTS) and it seems that the correct sequence is:

sudo apt-add-repository ppa:xtaran/conkeror
sudo apt-get update
sudo apt-get install conkeror conkeror-spawn-process-helper

The first step (apt-add-repository) seems to execute without a problem, giving the following output:

You are about to add the following PPA to your system:
Conkeror Debian packages for Ubuntu releases without xulrunner (i.e. for 11.04 Natty and later)
More info: https://launchpad.net/~xtaran/+archive/conkeror
Press [ENTER] to continue or ctrl-c to cancel adding it

Executing: gpg --ignore-time-conflict --no-options --no-default-keyring --secret-  keyring /tmp/tmp.Re7pWaDEQF --trustdb-name /etc/apt/trustdb.gpg --keyring /etc/apt/trusted.gpg --primary-keyring /etc/apt/trusted.gpg --keyserver hkp://keyserver.ubuntu.com:80/ --recv CB29CBE050EB1F371BAB6FE83BE0F86A6D689050
gpg: requesting key 6D689050 from hkp server keyserver.ubuntu.com
gpg: key 6D689050: "Launchpad PPA for Axel Beckert" not changed
gpg: Total number processed: 1
gpg:              unchanged: 1

However, the apt-get update seems unable to fetch packages from the newly added PPA, with its output ending in:

Hit http://security.ubuntu.com precise-security/restricted Translation-en
Hit http://security.ubuntu.com precise-security/universe Translation-en
Err http://ppa.launchpad.net precise/main Sources                    
404  Not Found
Ign http://extras.ubuntu.com precise/main Translation-en_US
Err http://ppa.launchpad.net precise/main i386 Packages
404  Not Found
Ign http://extras.ubuntu.com precise/main Translation-en
Ign http://ppa.launchpad.net precise/main Translation-en_US
Ign http://ppa.launchpad.net precise/main Translation-en
W: Failed to fetch http://ppa.launchpad.net/xtaran/conkeror/ubuntu/dists/precise  /main/source/Sources  404  Not Found

W: Failed to fetch http://ppa.launchpad.net/xtaran/conkeror/ubuntu/dists/precise/main/binary-i386/Packages  404  Not Found

E: Some index files failed to download. They have been ignored, or old ones used instead.

Accordingly, apt-get-install conkeror fails with:

mperdikeas@mperdikeas:~$ sudo apt-get install conkeror
Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package conkeror

Any ideas what might be wrong ?